The present invention relates to an angular reference apparatus for measuring angular velocity using gyroscopic or Coriolis effect on a vibratory inertial body, and particularly to a new type of vibrating Coriolis gyro sensor.
As strapdown inertial navigation technology continues to progress, there is a need to develop Coriolis gyro sensors, or gyroscopes, which have high accuracy, low cost, small size, high bandwidth, and rapid reaction.
UK Patent application 2 113 842 by Langdon (1983) describes a dual input-axis Coriolis gyro sensor with one or two wheels, or inertia-members, that are angularly dithered about their rotational axis of symmetry. Although, according to Langdon, a single inertia-member is sufficient in principle, two counter-rotating inertia-members are required to make the sensor insensitive to externally applied vibrations. In addition to the external vibration sensitivity, an instrument with a single inertia-member also transmits considerable reaction forces to the instruments attachment points, where appreciable loss of energy takes place.
Coriolis gyro sensors with two counter-rotating inertia-members are however very difficult and expensive to fabricate because they require careful and time-consuming balancing procedures of both inertia-members. In principle, the two counter-rotating inertia-members must move with exactly the same amplitude and with exactly opposite phase in order for the reaction moments to cancel. In practice, additional mechanisms are required to insure proper amplitude and phase control.
As noted, a Coriolis gyro sensor with a single inertia-member looses appreciable energy, which must be re-supplied to the inertia-member. Energy is re-supplied to the inertia-member by means of a force or torque, which causes bias drift errors because of unavoidable phase and magnitude errors.
It is therefore a need for high accuracy Coriolis gyro sensors with a single inertia-member, in which energy losses are kept at a minimum. It is an object of this invention to reveal a new, highly accurate, single or dual input-axis vibratory Coriolis gyro sensor with a single inertia-member in which energy loss and sensitivity to external vibrations are minimized by elastically connecting the inertia-member to a flexibly supported counter-inertia, which counteract, or balances the reaction forces from the inertia-member.
Boxenhorn (U.S. Pat. No. 4,598,585) describes a single-axis gyro sensor, which is comprised of an outer frame, which is torsion spring supported about the y-axis. The outer frame in turn supports an inner plate (or inertia-member) with a pair of torsion springs, allowing the inner plate to flex around the x-axis. The inner plate carries on it a substantial mass, which acts as a gyroscopic proof-mass. Both the outer frame and the inertia-member are designed to resonate at the same frequency. The frame is dithered by electrostatic forces at its resonant frequency. An input rate of the sensor around the z-axis, causes the dither oscillation of the frame about the y-axis to excite the inner plate such that it vibrates about the x-axis, due to Coriolis forces. This vibration is detected by a set of capacitive sensors attached to the top of the inner plate.
Boxenhorn further teaches that the sensor may be made of one of several combinations of materials. The flexures may for instance be made of silicon dioxide, silicon nitride or silicon-oxy-nitrides which is deposited (or implanted) on one side of a silicon sheet. During the fabrication of the sensor, the deposited or implanted material, which is used both as an etch stop and as a material for the flexures, exhibit shrinkage or swelling relative the silicon sheet. Because of differences in thermal expansion and the built-in stress in the flexures, the resonant frequency of the frame and the inner element deviate from each other as the temperature is changed. This frequency deviation causes an undesirable change of gyro sensitivity over temperature.
Bernstein (U.S. Pat. No 5,203,208) describes a gyroscope with two weights attached to a spring-supported inertia-member. The inertia-member is made from a sheet of silicon, which is doped with boron in a thin layer near one surface. The boron doped part of the silicon sheet act as a convenient etch-stop during manufacturing and also serves as the material for the flexures. The resonant frequencies of the inertia-member are intended to be identical about the x- and y-axis. As temperature changes, the two frequencies deviate from each other due to a change in the built-in stress and different thermal expansion of the boron-doped silicon material used in the flexures as compared to the sheet material. Bernstein uses flexible slots to minimize the change of stress in the flexures over temperature. The flexible slots decrease the stiffness in the axial direction along the z-axis, which is highly undesirable. The fabrication processes disclosed in prior art devices require some form of etch stop material to delineate the flexures. Such materials have much less stability than pure silicon.
In the two inventions by Boxenhorn and Bernstein described above, the stress in the hinge material is difficult to control and the resulting resonant frequencies of the proof-mass are unpredictable, which cause the instruments to exhibit uncontrollable output errors.
The instant invention avoids this problem by the use of a unitary material for the manufacturing of the sensor. As shown in FIGS. 2 and 3, the flexible beams have the same thickness as the frame, the counter-inertia and the inertia-member. The inventor has found that a Coriolis gyro sensor with uniform thickness can be easily fabricated by etching from a single sheet of high purity silicon. The etching terminates when the base material is etched through, which obviates the need to use a dopant such as boron, or an embedded oxide layer, as an etch stop to define the flexures. The invention by Boxenhorn described above is extremely sensitive to linear vibrations in the plane of the sensor because the flexures are offset from the center of gravity along the z-axis. For example, linear vibrations at or near the dither frequency and directed along the x-axis, cause an oscillatory moment to develop that is indistinguishable from a Coriolis moment. Such vibrations hence cause large output signals from the gyro, even in the absence of external input rate. The instant invention avoids this problem by utilizing flexures that are coincident with the center of gravity.